Ahoy Admiral S!
Flip 66: I’m the guy AdmiralS is referring to.
I feel your pain.
if you can see the tank on a classic 36 you may be able to start cutting it out in pieces and fitting multiple new tanks in that space. The good news for you is that aluminum can be easily cut apart. You may have to stack tanks and connect them through pipe.
You may be able to pour something into the aluminum tank to seal the pinholes.
If you can get to one side of the tank you may be able to cut a hole into the tank to inspect. If you can cut a hole open and identify where the leak is coming from, provided it’s only one pinhole, you may be able to seal it with either JB water weld from the inside. (careful, it’s only malleable for about five minutes ) or A better product to use would be Belzona pn 1111. ( workable for about 20 minutes)
I haven’t been so lucky.